A thrust piston pump apparatus of this kind is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 8-144948. In the thrust piston pump apparatus disclosed in the publication, a piston is assembled into a cylinder in such a manner as to be rotatable and to be able to reciprocate along a cylinder axis. The piston is configured to be driven by an electric motor. A rotary shaft, which is rotatably driven by the electric motor, is inserted into the piston in such a manner as to transmit rotation to the piston while allowing the piston to move axially.
In the thrust piston pump apparatus described in the above-mentioned publication, the piston and the electric motor are arranged in series along the cylinder axis. Thus, the thrust piston pump apparatus has a long configuration along the cylinder axis. A problem to tackle for the thrust piston pump apparatus is to reduce the length along the cylinder axis. The thrust piston pump apparatus can generate a high output in relation to pump volume by means of increasing an output torque of the electric motor. However, in this case, the outside diameter of the electric motor increases; consequently, the size of the pump also increases radially. Therefore, a large installation space is required.